RECENT PROGRESS IN PHYSICS. 



373 



if the openings were well secured with cement, it sufficed merely to 



lay a piece of the cliloride of calcium in tlie vessel. 



The comhination of this apparatus with the condensor gave rise to 



peculiar difficulties ; if a collector he screwed to the top instead of the 



knoh X, and the condensor plate placed on this, the electricity of the 



gold leaves Avill be drawn mainly into the collector plate when the 



condensing plate is touched, so that the leaves will diverge as soon as 



the condensor is raised, even if not the least electricity has been im- 

 parted to the collector. 



This circumstance renders the use of the condensor, in the ordinary 



manner, altogether uncertain. Andriessen remedied this defect in 



the following way : 



He placed the condensor, not upon the electroscope, but beside it. 



A glass tube fastened to a board and coated inside and outside with 



shellac, carried the collector as shown in figure 11. 



When the electricity is condensed on the collector, the condensor is 



raised, and the collector 



with its free electricity is 



brought into connexion 



with the electroscope by 



means of the wire x q. 



This wire is of soft cop- 

 per, and is wound around 



the stem of the knob x, 



which sustains the gold 



leaves ; w is a stick of 



shellac fastened to the 



wire, serving as a handle 



to bring the end q of the 



wire into contact with the 



collector. 



§ 13. Impkovemexts in Coulomb's torsion balance. — Since the time 

 of Coulomb, the electric torsion balance has been used by very few 

 and with but little effect; complaints were made of the uncertainty of 

 the instrument, and of its difficult management ; the opinion spread 

 abroad that exceedingly great^skill was required in the experiments 

 to produce reliable results with it. 



Riess opposed this prejudice (Pogg. Ann. LXXI, 359 ;) he experi- 

 mented much with this balance, and proved that, if the necessary care 

 has been taken in the construction of the instrument, the result will 

 not only be certain, but excessive skill in its use will not be required. 

 The instrument which Riess describes in the above mentioned com- 

 munication has the dimensions of the smallest balance which Coulomb 

 used in his measurements. The lower glass cylinder is one foot in 

 diameter and one foot high; the tube in which the metallic thread 

 hangs is fifteen inches long. The torsion balance of Riess is con- 

 structed on the same principle as that of Coulomb, but a few arrange- 

 ments are introduced which render a greater accuracy of observation 

 possible ; thus the position of the moveable arm or beam is observed 

 with a microscope. To make the most minute changes in the torsion 

 of the metallic wire, a micrometer screw is placed at the head of the 

 apparatus, and is turned by means of a dependent handle with a 



